Prominent Democrats in Congress reacted angrily today to disclosures that State Department investigators made apparently unauthorized offers of immunity to Blackwater security guards in the case of last month’s deadly shooting of 17 Iraqi civilians.

Calling the offers of immunity “an egregious misjudgment,” Representative Henry A. Waxman of California wrote Secretary of State Condoleezza Rice and demanded details about the offers, which he said “raises serious questions about who conferred the immunity, who approved it at the State Department, and what their motives were.”

Most of the guards who took part in the episode were offered what officials described as limited-use immunity, which means that they were promised they would not be prosecuted for anything they said in their interviews with the authorities as long as their statements were true.

Some government officials are calling these offers of immunity a potentially serious investigative misstep that could complicate efforts to prosecute the company’s employees involved in the episode.

Mr. Waxman is chairman of the Committee on Oversight and Government Reform, which has broad investigatory powers that Mr. Waxman has relished using on the Bush administration since Democrats won control of Congress.

Senator Patrick J. Leahy of Vermont, the chairman of the Senate Judiciary Committee and head of the appropriations subcommittee that has budget jurisdiction over the State Department, also issued an angry statement. “In this administration, accountability goes by the boards,” he said. “If you get caught, they will give you immunity. If you get convicted, they will commute your sentence.”

Mr. Waxman asked Ms. Rice when she and other top officials of the State Department first learned that the Blackwater people had been offered immunity. Mr. Waxman asked that details of the episode be made available to his panel by noon Friday.

The White House referred questions about the affair to the State Department, whose chief spokesman, Sean McCormack, responded cautiously to questions.

“The Department of State cannot immunize an individual from federal prosecution, federal criminal prosecutions,” Mr. McCormack said after conferring with department lawyers.

“I can’t in any way, shape or form comment on any arrangements arranged, arrived at with individuals concerning that case,” Mr. McCormack said. But he said he has heard nothing about the episode so far that would preclude prosecution.

The State Department investigators from the agency’s investigative arm, the Bureau of Diplomatic Security, offered the immunity grants even though they did not have the authority to do so, government officials said Monday. They spoke on the condition of anonymity because the investigation is ongoing. Prosecutors at the Justice Department, who do have such authority, had no advance knowledge of the arrangement, they added.

Most of the guards who took part in the episode were offered what officials described as limited-use immunity, which means that they were promised they would not be prosecuted for anything they said in their interviews with the authorities as long as their statements were true.

The immunity offers were first reported Monday by the Associated Press.

“If there’s any truth to this story, then the decision was made without consultation with senior officials in Washington,” one State Department official had said.on Monday

A spokeswoman for Blackwater, Anne E. Tyrell, said: “It would be inappropriate for me to comment on the investigation.”

The immunity deals came as an unwelcome surprise at the Justice Department, which was already grappling with the fundamental legal question of whether any prosecutions could take place involving American civilians in Iraq.

Blackwater employees and other civilian contractors cannot be tried in military courts and it is unclear what American criminal laws might cover criminal acts committed in a war zone. Americans are immune from Iraqi law under a directive signed by the United States occupation authority in 2003 that has not been repealed by the Iraqi parliament.

A State Department review panel sent to investigate the shootings concluded that there is no basis for holding non-Defense Department contractors accountable under United States law and urged Congress and the administration to urgently address the problem.

Earlier this month, the House overwhelmingly passed a bill that would make such contractors liable under a law known as the Military Extraterritorial Jurisdiction Act; the Senate is considering a similar measure.

The government has transferred the investigation from the diplomatic service to the Federal Bureau of Investigation, which has begun re-interviewing Blackwater employees without any grant of immunity in an effort to assemble independent evidence of possible wrongdoing.

Richard J. Griffin, the chief of Diplomatic Security Service resigned last week, in a departure that appeared to be related to problems with his supervision of Blackwater contractors.

In addition, the Justice Department reassigned the investigation from prosecutors in the criminal division who had read the State Department’s immunized statements to prosecutors in the national security division who had no knowledge of the statements.

Such a step is usually taken to preserve the government’s ability to argue later on in court that any case it has brought was made independently and made no use of information gathered under a promise that it would not be used in a criminal trial.

Immunity is intended to protect the Fifth Amendment right against self-incrimination while still giving investigators the ability to gather evidence. Usually people suspected of crimes are not given immunity and such grants are not given until after the probable defendants are identified. Even then, prosecutors often face serious obstacles in bringing a prosecution in cases in which defendants have been immunized.

The courts have made it all but impossible to prosecute defendants who have been granted immunity since the appellate court reversals of the Iran-contra affair convictions against John M. Poindexter, a former national security adviser, and Oliver L. North, a national security aide, who had each been immunized by Congress.

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